A flowmeter arranged in the supply path of a fluid (gas) for measuring the amount of supply of the fluid is generally so configured as to measure the instantaneous flow rate of the fluid by using a flow sensor and to integrate the measured flow rate. The integrated flow rate measured by the flowmeter is used as information on the basis of which the user is charged for the supply of the fluid, namely, for the amount of the fluid (gas) used. Thus, it is essential that the flowmeter should be able to measure the fluid supply amount with accuracy.
Depending on the location where the flowmeter is installed, however, it is occasionally the case that the fluid (gas) fluctuates even though the fluid is actually not in use. For example, after flowing regularly in one direction, the fluid (gas) slowly flows backward in the opposite direction. Such fluctuation of the fluid is caused by pressure imbalance between the opposite sides of the flowmeter due to conditions of use of the fluid (gas) in other supply systems, or by uneven distribution of temperature in the piping. Since the flowmeter merely integrates the instantaneous flow rate measured by the flow sensor, a problem arises in that the flow rate of the fluctuating fluid is repeatedly added up.
Conventionally, therefore, check valves are provided on the opposite sides of the flowmeter, for example. The use of the valves, however, makes the construction of the flowmeter complicated and also leads to increase in manufacturing cost. Also, a proposal has been made that in a low flow rate region where fluctuation of the fluid takes place, the integration of measured flow rates should be suspended. Where the fluid is not fluctuating but just flowing at a very small rate, however, the flow rate of the fluid fails to be measured, entailing increase in measurement error.
There has also been proposed a method in which the instantaneous flow rate of a fluid, measured by a flow sensor, is added or subtracted, depending on the flowing direction of the fluid, over a predetermined time, the resultant positive value is added to the totalized flow rate only if the positive value is greater than a preset threshold, and the resultant negative value is reset if the negative value is greater than a preset threshold (see, e.g., Patent Document 1).
Patent Document 1: Unexamined Japanese Patent Publication No. 2002-81978
In this method, however, in order to cancel out fine changes in the flow rate of the fluid in the forward and backward directions over the predetermined time, a cumulative sum of the instantaneous flow rates measured by the flow sensor is not added to the totalized flow rate until the cumulative sum exceeds the preset threshold. A problem therefore arises in that the minimum metering unit (integration unit) for the flow rate integration is determined by the threshold. In cases where the fluid fluctuates in the forward and backward directions in large quantities and at the same time the fluctuation takes places at long intervals of time, the threshold needs to be set to a relatively large value, making the minimum metering unit even larger. Further, it is necessary that the forward and backward fluctuation of the fluid be canceled out over a long period of time corresponding to the fluctuation interval, giving rise to a problem that the interval for the flow rate integration is also prolonged.